The present invention relates to a CCD type solid-state image sensor, and more particularly to a solid-state image sensor having a buried transfer gate channel region and a rivet type photodiode region with a stepwise concentration distribution.
FIG. 1 is a sectional view of a conventional interlaced CCD type solid-state image sensor.
As shown in FIG. 1, the conventional solid-state image sensor comprises a n type substrate 11, a p type well 12 formed in the n type substrate 11, a plurality of n type photodiode regions 13 formed in the p type well 12 by an implantation of n type impurity ions to have a depth of 0.5 to 0.7 .mu.m, and a p.sup.+ type layer 14 of a high concentration formed over each n type photodiode 13 to have a depth of 0.1 to 0.2 .mu.m so as to reduce a surface noise of the n type photodiode 13. The solid-state image sensor further comprises a plurality of n type vertical CCD (VCCD) channel regions 15 each formed in the p type well 12 to a depth of 0.3 to 0.7 .mu.m to be spaced from each corresponding n type photodiode region 13 and adapted to serve as a vertical transfer channel for a signal transfer, a p.sup.- type transfer gate channel region 16 formed in the p type well 12 between each n type VCCD channel region 15 and each corresponding n type photodiode region 13, a p type channel stop region 17 formed in the p type well 12 between each n type photodiode region 13 and each n type VCCD channel region 15 neighboring to the n type photodiode region 13, a thin insulating film 18 formed over the n type substrate 11, a transfer gate 19 formed on the insulating film 18 over each p.sup.- type transfer gate channel region 16 and each n type VCCD channel region 15 and made of a polysilicon film doped with n type impurity ions, a photoshield metal film 21 formed over the entire surface of the resulting structure except for a region disposed above each n type photodiode region 13 to form an opening 22 through which light beams enter the n type photodiode region 13, and an interface insulating film 20 for insulating the transfer gate 19 from the photoshield metal film 21.
The conventional interlaced CCD type solid-state image sensor having the above-mentioned structure operates in an integration mode for accumulating signal charges generated from each n type photodiode region 13 by light beams introduced through each corresponding opening 22 or in a read-out mode for transferring the signal charges accumulated in the n type photodiode region 13 to the corresponding n type VCCD channel region 15.
The integration mode operation is the operation for collecting signal charges for a predetermined time. Potential distribution for various parts of the solid-state image sensor in the integration mode is shown in FIG. 2 which is taken along the line A-A' of FIG. 1. Due to a potential barrier formed by each p.sup.- type transfer gate channel region 16 and the p type well 12, the signal charges generated by the incident light beams are accumulated in each corresponding n type photodiode region 13 for a predetermined time of, for example, 1/60 to 1/30 second.
In the integration mode, a voltage of 0 V is applied to the transfer gate 19. As a result, each p.sup.- type transfer gate channel region 16 forms surely a potential barrier for preventing the charges accumulated in each corresponding n type photodiode region 13 from being moved, as shown in FIG. 2.
On the other hand, the read-out mode operation is the operation for transferring the signal charges accumulated in each n type photodiode region 13 to each corresponding n type VCCD channel region 15. Potential distribution for various parts of the solid-state image sensor in the read-out mode is shown in FIG. 3 which is taken along the line A-A' of FIG. 1 and in FIG. 4 which is taken along the line B-B' of FIG. 1.
In the read-out mode, a voltage of 15 V is applied to the transfer gate 19. As a result, the potential of each n type VCCD channel region 15 becomes very low, as shown in FIGS. 3 and 4. Also, the potential of each p.sup.- type transfer gate channel region 16 becomes low. Accordingly, the signal charges accumulated in each photodiode region 13 move to each corresponding n type VCCD channel region 15 through each corresponding p.sup.- type transfer gate channel region 16. Thus the signal charges can be read out.
However, the conventional solid-state image sensor has a drawback that the surface area of each transfer gate 19 should be large because the signal charges accumulated in each photodiode region 13 move to each corresponding n type VCCD channel region 15 through each corresponding p.sup.- type transfer gate channel region 16. In the conventional solid-state image sensor, the probability that noise charges are included in the signal charges transferred to the VCCD channel regions 15 is adversely increased.
Since the junction depth of each photodiode region 13 is small, the sensitivity characteristic of the solid-state image sensor is poor. This poor sensitivity characteristic results in a poor smear characteristic.
Where each photodiode region 13 has a large width, the image lag characteristic of the solid-state image sensor is poor because effect of a fringing field is small even though each transfer gate 19 is turned on.
Moreover, when strong light beams enter photodiodes 13 through openings 22, charge paths for guiding excessive charges to the substrate 11 become narrower. As a result, the blooming characteristic is degraded.
FIG. 5 is a sectional view of another conventional solid-state image sensor having a double well structure exhibiting an improved blooming characteristic. As shown in FIG. 5, a second p type well 23 is formed beneath each n type VCCD channel region 15. The formation of the second p type well 23 is achieved by implanting p type impurity ions in a portion of the p type well 12 disposed beneath each n type VCCD channel region 15 by use of the high energy ion implantation process.
This conventional solid-state image sensor is disclosed in ITEJ Technical Report Vol 16, No. 18, pp 7-12, IPU 92.8-92.9 (Feb, 1992).
Although the conventional solid-state image sensor having the double well structure improves the blooming characteristic over the case of FIG. 1, it can not solve the problem caused by the signal charge movement through the photodiode regions having the small junction depth and the transfer gate channel regions, as in the case of FIG. 1.